The prior art has dealt extensively with the subject of isolation, purification and improvement of the nutritional quality and flavor of soybean protein. Soybean protein in its native state has impaired nutritional quality due to the presence of phytic acid complexes which interfere with mammalian mineral absorption, and the presence of antinutritional factors including trypsin inhibitors which interfere with protein digestion in mammals.
The present invention provides a method of preparing an improved soy protein isolate having exceptionally low phytate and aluminum content. The aluminum content of soy based infant formulas is a significant concern. Infant nutritional formulas containing soy protein isolate produced using the methods disclosed herein are more nutritionally desirable since reduction of the aluminum content provides soy formula compositions more similar to human breast milk.
In a typical commercial process, the soy proteins are extracted at slightly alkaline pH from defatted soy flake or defatted soy flour. The major protein fraction is then precipitated from the clarified extract by adjusting the pH to the isoelectric point of the proteins (pH 3.8 to 6.0). Inasmuch as the proteins are insoluble at this pH the protein curd can be separated from soluble sugars, salts, etc., by centrifugation. To complete the purification, the protein curd is washed with water at least once at this isoelectric pH, then the protein is spray-dried either as is or after resuspension at neutral pH. Under such prior art conditions, a major portion of the phytate present in the soy flour will complex with the protein and will be present in the soy protein isolate. Commercial soy protein isolates typically have a phytate content of 2.0-2.5% and in some instances as much as 3% by weight.
In contrast to the prior art the present invention raises the pH of the aqueous slurry of soy flour to about 9.0. As used herein and in the claims "aqueous slurry" is understood to mean a slurry comprising more than 50% water by weight. After a suitable time for extraction the material is circulated through an ultrafiltration device that will pass the protein and reject fiber, aluminum and phytate. At the completion of the ultrafiltration process the pH of the permeate is adjusted to the isoelectric point of soy protein. The purified protein then precipitates, and the precipitated material is collected by centrifugation.
Filtration is a much used technique for separating wanted substances from those which are unwanted. There are two customary ways in which a feed stream meets a filter: dead-end filtration and cross flow filtration. In dead-end filtration, the feed stream flows perpendicular to the filter membrane. In cross flow filtration, on the other hand, the feed stream runs parallel to the filter membrane and the filtrate diffuses across it. The product which passes through the membrane is known as the "permeate" and the product which is retained is known as the "retentate". Filters are often classified by retained particle size. For example, membrane microfilters generally retain particles having a diameter greater than approximately 0.1-10 microns in diameter, ultrafilters generally retain particles and macromolecules having a diameter greater than approximately 0.05-0.1 microns and hyperfilters generally retain molecules having a diameter greater than approximately 0.001 microns. Put in another way, ultrafilters generally retain particles having molecular weights in excess of 10,000 to 500,000. In the laboratory, filter selection is usually straight forward, but scale-up to industrial applications often presents numerous difficulties.
In theory, ultrafiltration should permit the selective separation, concentration, and purification of protein components. In practice, ultrafiltration does not proceed according to ideal hypotheses. For example, most current ultrafiltration membranes have variable pore diameters and their molecular weight cut-off capacity is not uniform. Furthermore, the permeate flux (volume of product per unit of filter area per unit of time) of an ultrafiltration membrane is greatly affected by the presence of a polarization layer or by fouling of the membrane.
Polarization layers form in the course of ultrafiltration and modify the transfer of solutes across the membrane, thereby lowering the permeation rate of the device and changing its separation characteristics. Polarization is caused by convection through the membrane. If fluid flows through the membrane faster than the retained material can diffuse back into the retentate fluid, a saturated layer builds up next to the membrane. The layer's depth and its resistance to flow depend on the speed at which the retentate is circulated. The total permeability of the membrane in the course of operation depends on the polarization layer's thickness and also the nature of its components.
The resistance due to fouling builds as deposits chemically bind to the membrane. Fouling is quite distinct from polarization, in which the interfering layer is held against the membrane by hydrodynamic (not chemical) forces.
To obtain highly purified concentrates, filtration may be followed or accompanied by diafiltration which consists of adding additional fluid to the retentate concentrates and subjecting them to another filtration cycle. Diafiltration may be a batch process (dilutions followed by successive concentrations) or a continuous process (water is added at the same rate as the permeate is eliminated).
The present invention relates to an improved ultrafiltration process for separating protein from undesirable components such as phytates and aluminum. This is accomplished by using metallic ultrafiltration membranes at a pH at which the protein will be soluble and pass through the membrane while phytates and aluminum are retained.
The prior art does not show a simple one step removal of phytate and aluminum via ultrafiltration.